Receptors to which physiologically active substances bind may serve as drug development targets. Particularly, receptors that alter intracellular cAMP concentration as second messengers, such as G protein-coupled receptors (hereinafter, also referred to as GPCRs), are important as drug development targets. Many agents that bind to such receptors have been developed so far. Examples of such GPCRs include receptors to which arginine vasopressin (hereinafter, also referred to as AVP) or glucagon-like peptide-1 (hereinafter, also referred to as GLP-1) binds.
Arginine vasopressin produced by the pituitary gland is a peptide hormone with a molecular weight of 1000 that is composed of 9 amino acids. AVP is one type of important water and electrolyte regulating hormone that performs water reabsorption in the kidney depending on decrease in circulating blood volume, a rise in plasma osmolality, etc. The assay of AVP in plasma is clinically applied widely to, for example, the diagnosis of disorders of water and electrolyte metabolism, such as diabetes insipidus which is a pathological condition involving the decreased secretion of AVP or the syndrome of inappropriate AVP secretion in which increase in AVP secretion is instead seen. Moreover, AVP is not only assayed for endocrine disease but also used as a marker for the prognosis of small-cell lung cancer or for acute heart failure. An ELISA kit by which chemically labeled AVP is allowed to compete with AVP in a sample and AVP is quantified on the basis of a competitive inhibition rate is widely used as a method for assaying AVP. However, the ELISA method using the competition technique has poor sensitivity and has limitations in the assay of a microscale sample in blood. Since AVP is a low-molecular-weight peptide, AVP in plasma cannot be assayed with high sensitivity due to the unavailability of sandwich ELISA assay using sandwich antibodies against a plurality of antigen determinants that permit higher sensitivity. Thus, a radioimmunoassay method using isotope-labeled AVP, which was developed by Robertson et al. in 1973, is still used widely in clinical practice as a method for assaying AVP in plasma (Non Patent Literature 1).
A V1a receptor, a V1b receptor and a V2 receptor (hereinafter, also referred to as V2R) are known as AVP receptors. A selective vasopressin V2 receptor antagonist tolvaptan has been reported to be effective for hyponatremia (heart failure, liver cirrhosis and the syndrome of inappropriate antidiuretic hormone secretion (SIADH)) (Non Patent Literature 2).
Incretin, one type of gastrointestinal hormone, is secreted from the intestinal tract in response to the stimulation of dietary intake and enhances insulin secretion from pancreatic beta cells. GLP-1 and GIP (gastric inhibitory peptide or glucose-dependent insulinotropic polypeptide) are known as major active molecules of incretin hormones. Among them, the GLP-1 peptide has received the highest attention as a blood glucose regulator.
GLP-1 not only promotes insulin secretion dependent on glucose but also participates in the inhibition of glucagon secretion from pancreatic alpha cells and further in the inhibition of gastric emptying. Moreover, GLP-1 can act centrally on the hypothalamus to suppress appetite, resulting in the control of weight gain. Since this series of physiological actions brings about desirable effect on the treatment of type 2 diabetes mellitus, drug development aimed at enhancing the action of GLP-1 has been carried out widely.
GLP-1 is secreted from intestinal L cells and converted to GLP-1 (7-37) or GLP-1 (7-36 amide) by processing. Both the forms have equivalent physiological activity against GLP-1 receptors. In blood, 80% or more of active forms of GLP-1 are the GLP-1 (7-36 amide) forms. These GLP-1 peptides are called “active forms of GLP-1”. The active form of GLP-1 is immediately inactivated into GLP-1 (9-37 or 9-36 amide) through cleavage of its N-terminal moiety by dipeptidyl peptidase-IV (DPP-IV). In this regard, DPP-IV inhibitors aimed at enhancing the action of GLP-1 have been developed and clinically used widely (Non Patent Literature 3). Furthermore, GLP-1 derivatives resistant to DPP-IV have also been developed and come into action as clinical drugs. Moreover, agents targeting the action of enhancing GLP-1 secretion are also under clinical trial as next generation antidiabetic drugs (Non Patent Literature 4). Increase in demand in the near future for a method for accurately and rapidly measuring the serum concentration of the active form of GLP-1 is expected in the determination of the effect of these GLP-1 action enhancing drugs.
Moreover, the fasting serum concentration of the active form of GLP-1 has recently been reported to be much lower in Asians than in Westerners, suggesting that a highly sensitive assay method capable of detecting difference in the serum concentration of the active form of GLP-1 among races or among individuals will be required (Non Patent Literature 5).
An EIA (Enzyme Immuno Assay) kit based on a two-antibody sandwich method has already been launched as a method for measuring the serum concentration of the active form of GLP-1 before undergoing degradation. The existing kit is used in, for example, the determination of the effect of DPP-IV inhibitors that inhibit the degradation of GLP-1. The lowest detection sensitivity of the existing EIA method is approximately 1-2 pM (3.3-6.6 pg/ml) for the active form of GLP-1 (7-36 amide) and is also the same level for the GLP-1 (7-37) form. Since the fasting concentration of the active form of GLP-1 is 1 pM or lower in Asians, higher sensitivity is required for accurately measuring the fasting concentration of the active form of GLP-1. In this respect, the concentration and purification of a sample are necessary for the existing kit (Non Patent Literature 6).